The present invention relates to radar systems and methods, and more particularly to a frequency-agile radar system capable of detecting and identifying a particular type of target from a safe stand-off distance.
The word "radar" is an acronym for "radio detecting and ranging." Most radar systems emit electromagnetic radiation in the radio frequency (RF) range in short bursts or pulses, with a relatively long interval between pulses. Then, the pulses which have been re-radiated (reflected) from objects (targets) are detected. The time delay between the transmitted and received pulses provides a measure of the distance to the target. The bearing to the target is usually determined from a knowledge of the pointing direction of a narrow RF beam produced by the radar antenna.
Early radar systems used radar frequencies having wavelengths of several meters. By the 1940's, microwave frequencies were used, and now there are radars operating at millimeter and optical wavelengths. The advantage of the shorter wavelengths is a narrower beam width for a given overall antenna size. The disadvantages of shorter wavelengths include greater loss of signal strength because of increased atmospheric absorption and scattering plus various equipment limitations.
The civilian and military applications of radar for surveillance, navigation, air-traffic control, weather tracking, early warning, and missile guidance are well known. Perhaps not so well known, but still a viable and valuable application for radar systems, are systems that provide radar imagery. Radar imagery is complementary to photographs obtained with infrared, visible, or ultraviolet waves. Although similar to a photograph, radar imagery highlights different features. For example, radar is sensitive to vertical dimensions and, therefore, it emphasizes topographic features. Since the atmosphere is essentially transparent for microwaves, radar thus permits acquisition of information under "all weather," day or night conditions that are impossible with photographic techniques. For example, radar imagery has permitted several million square kilometers of South America to be mapped, even though the mapped area is usually concealed by heavy cloud cover. Disadvantageously, however, radar imagery systems have yet been able to provide a meaningful discrimination between two similar-sized, yet different, objects that may be illuminated by the probing radar signal. That is, radar imagery systems have not generally been able to successfully discriminate at relatively long ranges (e.g., several kilometers) between, e.g., an automobile and a pile of garbage, or between a microwave dish antenna and a tin roof on a storage shed. What is needed, therefore, is a means for not only detecting and mapping objects with a radar system, but also a means for discriminating between the different types of objects that are detected.
While the atmosphere is essentially transparent for microwaves, the earth is not. Thus, electromagnetic waves are highly attenuated (with the attenuation increasing with an increase in frequency) when propagating through the earth, due to moisture and dense objects. Even so, radar systems that provide ground probing at short ranges (a few meters) have proved useful for a number of civilian applications, such as (1) detection of buried pipes (metallic and nonmetallic) and utility cables; (2) archeological mapping and geophysical prospecting; (3) location of voids or other subsurface conditions under highways and bridges; (4) detection of hidden objects and voids in walls and tunnels; or (5) measurement of ice thickness and location of permafrost. There are also corresponding military applications for a ground-probing radar system, such as the location of mines or explosives that have been hidden underground, in tunnels, or elsewhere.
Typically, a ground-probing radar is portable or mobile, and uses an antenna mounted at or near the ground. It has a high-range resolution capability made possible through the use of an extremely wide bandwidth (e.g., several hundred megahertz). Disadvantageously, such ground-probing systems are only effective at a very short range, e.g., a few meters. For some types of objects, such as a dangerous underground void, or a buried land mine, it is necessary that the object be detected from a safe stand-off distance, e.g., before the void caves in due to the weight of the equipment and operator, or before the land mine detonates. Hence, what is needed is a ground-probing radar system that can successfully detect dangerous hidden conditions or objects from a safe stand-off distance.
Further, even at a relatively short range, e.g., several meters, it is important that a ground-probing radar system be able to successfully discriminate between two similar sized objects, one of which is dangerous and the other of which is benign. Thus, there is a need for a system and method that not only detects hidden objects from a safe stand-off distance, but that also discriminates between a dangerous object, e.g., a buried land mine, and a benign object that has a similar size and shape, e.g., a buried can or a rock, regardless of the orientation of the object relative to the probing signal.
The present invention advantageously addresses the above and other needs.